Estimated beam pattern and echolocation characteristics of clicks recorded from a free-ranging Ganges river dolphin

Recordings of bio-sonar clicks from a free-ranging Ganges river dolphin (Platanista gangetica) in Budhabalanga river in Orissa, India have been made for the first time in April 2006. A high frequency multi-hydrophone 3.2 meter long array composed of three hydrophones forming an equispaced linear array and another two hydrophones in conjunction with the central hydrophone forming a small SSBL triangular array in a plane perpendicular to the array axis was used. The array structure was deployed both in horizontal and vertical configurations. The click sounds of the dolphin received on each hydrophone were sampled at a rate of 500 kHz per channel and recorded over a period of 2 days. An estimate of the dolphin's 3-D location was made using the relative delays of the click signal at the hydrophones. From these sampled click data, the sonar beam pattern of the animal (in both horizontal plane and vertical plane) has been estimated using difference of ASL (apparent source level of the dolphin) and the absolute angles of the dolphin location between the central hydrophone and the other two peripheral hydrophones for the "on axis clicks". The dolphin made several "runs" towards the array during the observation period. The variation of ASL with the range of the dolphin during its approach towards the array showed a systematic trend of reduction in ASL as the range reduced. This suggests that the animal was echolocating on the array as a possible target. The results also suggest that the animal can exercise control of the transmit signal level.